Process and apparatus for the gas-phase polymerization of alpha-olefins

ABSTRACT

Process for gas-phase polymerization carried out in two interconnected polymerization zones, to which one or more α-olefins CH 2 =CHR are fed in the presence of catalyst under reaction conditions and from which the polymer product is discharged. The process is characterized in that the growing polymer flows through a first polymerization zone under fast fluidization conditions, leaves said first zone and enters a second polymerization zone through which it flows in a densified form under the action of gravity, leaves said second zone and is reintroduced into the first polymerization zone, thus establishing a circulation of polymer around the two polymerization zones. The novel process allows olefins to be polymerized in the gas phase with high productivity per unit volume of the reactor without incurring the problems of the fluidized-bed technologies of the known state of the art.

[0001] The present invention relates to a process for the gas-phasepolymerization of olefins carried out in two interconnectedpolymerization zones, to which one or more α-olefins CH₂=CHR are fed inthe presence of a catalyst under polymerization conditions and fromwhich the produced polymer is discharged. In the process of the presentinvention the growing polymer particles flow through a firstpolymerization zone under fast fluidization conditions, leave said firstzone and enter a second polymerization zone through which they flow in adensified form under the action of gravity, leave said second zone andare reintroduced into the first polymerization zone, thus establishing acirculation of polymer between the two polymerization zones.

[0002] The development of catalysts with high activity and selectivityof the Ziegler-Natta type and, in more and more applications, of themetallocene type, has led to the widespread use on an industrial scaleof processes, in which the polymerization of the olefins is carried outin a gaseous medium in the presence of a solid catalyst. Compared withthe more conventional technology in liquid suspension (of monomer or ofmonomer/solvent mixtures), this technology has the following advantages:

[0003] a) operational flexibility: the reaction parameters can beoptimized on the basis of the characteristics of the catalyst and of theproduct and are not limited by the physico-chemical properties of theliquid mixtures of the reaction components (generally including hydrogenas a chain transfer agent);

[0004] b) widening of the product range: the effects of swelling of thegrowing polymer particle and of solubilization of polymer fractions in aliquid medium greatly reduce the range of production of all the kinds ofcopolymers;

[0005] c) minimization of the operations downstream of thepolymerization: the polymer is obtained directly from the reactor in theform of dry solid and requires simple operations for removing dissolvedmonomers and deactivating the catalyst.

[0006] All the technologies devised hitherto for the gas-phasepolymerization of α-olefins provide for maintaining a bed of polymer,through which the reaction gases flow; this bed is maintained insuspension either by mechanical stirring (stirred-bed reactor) or byfluidization obtained by recycling the reaction gases themselves(fluidized-bed reactor). In both the reactor types, the monomercomposition around the polymer particle in the reaction is maintainedsufficiently constant owing to the induced stirring. Said reactorsapproximate very closely the ideal behaviour of the “continuousstirred-tank reactor” (CSTR), making it relatively easy to control thereaction and thereby ensuring consistency of quality of the product whenoperating under steady-state conditions.

[0007] What is by now the most widely established industrial technologyis that of the fluidized reactor operating under “bubbling” conditions.The polymer is confined in a vertical cylindrical zone. The reactiongases exiting the reactor are taken up by a centrifugal compressor,cooled and sent back, together with make-up monomers and appropriatequantities of hydrogen, to the bottom of the bed thorough a distributor.Entrainment of solid in the gas is limited by an appropriatedimensioning of the upper part of the reactor (freeboard, i.e. the spacebetween the bed surface and the gas offtake), where the gas velocity isreduced, and, in some designs, by the interposition of cyclones in theexit gas line.

[0008] The flow rate of the circulating gas is set so as to assure afluidization velocity within an adequate range above the minimumfluidization velocity and below the “transport velocity”. The heat ofreaction is removed exclusively by cooling the circulating gas. Thecatalyst components are fed in continuously. The composition of thegas-phase controls the composition of the polymer. The reactor isoperated at constant pressure, normally in the range 1-3 MPa. Thereaction kinetics are controlled by the addition of inert gases.

[0009] A significant contribution to the reliability of thefluidized-bed reactor technology in the polymerization of α-olefins wasmade by the introduction of suitably pretreated spheroidal catalyst ofcontrolled dimensions and by the use of propane as diluent (see WO92/21706).

[0010] Fluidized-bed technology has limits, some of which are discussedin detail below.

[0011] A) Removal of the heat of reaction.

[0012] The maximum fluidization velocity is subject to quite narrowlimits (which already entail reactor volumes for disengagement which areequal to or greater than those filled by the fluidized bed). Dependingon the heat of the reaction, the polymer dimensions and the gas density,a limit to the productivity of the reactor (expressed as hourly outputper unit reactor cross-section) is inevitably reached, where operationwith gas inlet temperatures higher than the dew point of the mixture ofthe gases is desired. This limit can lead to reductions in the plantoutput, in particular in the copolymerization of ethylene with higherα-olefins (hexene, octene), which is carried out with conventionalZiegler-Natta catalysts, requiring gas compositions rich in sucholefins.

[0013] Many ways of overcoming the limits, in terms of heat removal, ofthe traditional technology have been proposed, based on partialcondensation of the circulating gases and on the use of the latent heatof evaporation of the condensates for controlling the temperature in theinterior of the reactor (see EP-89691, U.S. Pat. No. 5,352,749, WO94/28032). Although technically worthy of consideration, all the systemsproposed for implementing the principle render the operation of thefluidized reactors critical.

[0014] In particular (and apart from problems associated with thedistribution of wet solids in the plenum below the distribution grid),the technology used in patents EP-89691 and U.S. Pat. No. 5,352,749relies on the turbulence generated by the grid to distribute the liquidover the polymer. Possible coalescence phenomena in the plenum can giverise to uncontrollable phenomena of poor distribution of liquid withformation of agglomerates which can not be redispersed, in particular inthe case of polymers which have a tendency to stick. The discriminationcriterion given in U.S. Pat. No. 5,352,749 reflects situations understeady-state conditions, but offers no feasible guide for situations ofeven a transient “reaction runaway”, which can lead to irreversible lossof fluidization, with a consequent collapse of the reactor.

[0015] The method described in patent WO 94/28032 involves separation ofthe condensates and their distribution above the grid by means ofspecial, suitably located nozzles. In fact, the condensates inevitablycontain solids in reactive conditions, whose concentration can becomevery high at low condensate amounts. Moreover, the inherent difficultyof uniformly distributing a suspension over a number of nozzles cancompromise the operability of some of them and a blocking in one nozzleadversely affects the distribution of the liquid evaporating in therelevant section of the reactor. It is also clear that the efficiency ofthe operation depends upon a vigorous circulation of solids in thereactor and, below the injection points, this is reduced by anunbalancing of the gas flow rates caused by large quantities ofcondensates. Furthermore, any need for maintenance on one nozzlerequires a complete shut-down of the reactor.

[0016] B) Molecular weight distribution

[0017] As already stated, a fluidized bed shows a behaviour directlycomparable with an ideally mixed reactor (CSTR). It is generally knownthat, in the continuous polymerization of α-olefins in a single stirredstage (which also involves steady composition of the monomers and of thechain transfer agent, normally hydrogen) with Ti catalysts of theZiegler-Natta type, polyolefins having a relatively narrow molecularweight distribution are obtained. This characteristic is even moreemphasized when metallocene catalysts are used. The breadth of themolecular weight distribution has an influence both on the Theologicalbehaviour of the polymer (and hence the processability of the melt) andon the final mechanical properties of the product, and is a propertywhich is particularly important for the (co)polymers of ethylene.

[0018] For the purpose of broadening the molecular weight distribution,processes based on several reactors in series, in each of which itbecomes possible to operate at least at different hydrogenconcentrations, have gained industrial importance. A problem typicallyencountered also with these processes, when a very broad molecularweight distribution is required, is an insufficient homogeneity of theproduct. Particularly critical is the homogeneity of the material inblow-moulding processes and in the production of thin films, in whichthe presence of even small quantities of inhomogeneous material bringsabout the presence of unfused particles in the film (“fish eyes”). Inpatent application EP-574,821, a system of two reactors is proposedwhich operate at different polymerization conditions with mutualrecirculation of polymer between the two. Even if the concept issuitable for solving the problem of the homogeneity of the product, asshown by the experimental results, such a system involves investmentcosts and a certain operational complexity.

[0019] In other cases, polymers of broad molecular weight distributionare obtained by the use of mixtures of different Ziegler-Natta catalystsin a single reactor, each catalyst being prepared so as to give adifferent response to hydrogen. It is clear that a mixture of granuleseach with its own individuality are obtained at the exit from thereactor. It is difficult to obtain homogeneity of the product by thisroute.

[0020] C) Discharge of the product

[0021] The technology of polymerizing α-olefins in gas-phase reactorshas rapidly developed in the last years, and the range of polymersobtainable in this way has widened greatly. In particular, besideshomopolymers of ethylene and propylene, a wide range of copolymers canbe produced industrially, for example:

[0022] random copolymers of propylene/ethylene,propylene/ethylene/higher α-olefins and propylene/higher α-olefins;

[0023] polyethylenes of low and very low density (LLDPE, VLDPE),modified with higher α-olefins containing 4 to 8 carbon atoms;

[0024] heterophasic copolymers of high impact strength, obtained bygrowth on the active centres of the catalyst, in successive stages, ofone or more of the polymers listed above and of ethylene/propylene orethylene/butene rubbers; and

[0025] EPR and EPDM rubbers.

[0026] In short, in the polymers producible in the gas phase, themodulus of flexibility varies from 2300 MPa to values lower than 100MPa, and the xylene-soluble fraction varies from 1% to 80%. Theflowability, compactability and sticking properties turn out to beextremely variable as a function of the degree of crystallinity, of themolecular weight and of the composition of the various polymer phases.Many of these products remain granular and flowable (and henceprocessible) as long as they are maintained in a fluidized state or influx, which are conditions under which the static forces between theindividual solid particles have no effect. They tend more or lessrapidly to clump together and to form aggregates if they are allowed tosettle or to be compacted in stagnant zones; this phenomenon isparticularly enhanced under reaction conditions where, due to thecombined action of the temperature and the large quantity of dissolvedhydrocarbons, the polymer is particularly soft, compressible andcompactable, and sticky. The characterization of soft and stickypolymers is efficaciously described in EP-348,907 or U.S. Pat. No.4,958,006.

[0027] The most direct solution for the discharge of the polymer fromthe reactor consists of a direct discharge from the fluidized bedthrough a controlled valve. This type of discharge combines simplicitywith the advantage of not producing stagnant zones. Where a sufficientlylow pressure (in the range 0.5 - 3 bar gauge) is maintained downstreamof the discharge valve, the reaction is virtually stopped either by thetemperature reduction due to the evaporation of the monomers dissolvedin the polymer or due to the low partial pressure of the monomers in thegas: in this way, any risk in the receiver equipment downstream of thereactor is avoided.

[0028] Nevertheless, it is known that the amount of gas discharged withthe polymer from a fluidized bed through an orifice reaches very highvalues as a function of the reactor pressure, of the fluidizationvelocity, of the density of the solids in the bed, etc. (see, forexample: Massimilla, “Flow properties of the fluidized dense phase”, in“Fluidization”, p. 651-676, eds. Davidson & Harrison, Academic, NewYork, 1971). High amounts of gas discharged with the polymer representboth investment costs and operating costs, it being necessary torecompress this gas in order to get back to the reactor pressure fromthe receiver pressure. In many industrial applications, discontinuousdischarge systems have thus been installed, with interposition of atleast two hoppers in alternating operation. For example, U.S. Pat. No.4,621,952 describes a discharge system in which the polymer istransferred intermittently and at high differential pressures from thereactor to a settling tank. The momentum of the polymer which, duringthe filling phase, impinges first on the walls of the settling tank andthen on the bed of polymer compacts the material which loses itsflowability properties. During the filling phase the pressure in thesettling tank rises rapidly to the value of the reactor pressure and thetemperature does not change significantly. The reaction proceedsadiabatically at high kinetics. With soft and sticky products, thiseasily leads to the formation of agglomerates which cannot begranulated, with consequent difficulties with the discharge to thereceiving tank below. Analogous observations apply to U.S. Pat. No.4,703,094.

[0029] The limits of the intermittent system are clearly revealed by theproposal for complicated continuous systems. Japanese patent JP-A-58032,634 provides for the installation of an internal screw in thereactor for compacting the polymer towards the discharge; U.S. Pat. No.4,958,006 proposes the installation of an extruder, the screws of whichare fed directly in the interior of the fluidized-bed reactor. Apartfrom the complication and the difficulty of industrial application, thesystems proposed are in any case altogether inadequate for feeding thepolymer to a subsequent reaction stage.

[0030] A novel polymerization process has now been found, and thisrepresents a first aspect of the present invention, which allows olefinsto be polymerized in the gas phase with high hourly output per unitreactor volume without incurring the problems of the fluidized-bedtechnologies of the known state of the art. A second aspect of thepresent invention relates to an apparatus for carrying out this process.

[0031] The gas-phase polymerization process of the present invention iscarried out in a first and in a second interconnected polymerizationzones to which one or more α-olefins CH₂=CHR, where R is hydrogen or ahydrocarbon radical having 1-12 carbon atoms, are fed in the presence ofcatalyst under reaction conditions and from which the polymer producedis discharged.

[0032] The process is characterized in that the growing polymerparticles flow through the first of said polymerization zones under fastfluidization conditions, leave said first polymerization zone and enterthe second of said polymerization zones through which they flow in adensified form under the action of gravity, leave said secondpolymerization zone and are reintroduced into said first polymerizationzone, thus establishing a circulation of polymer between the twopolymerization zones.

[0033] As is known, the state of fast fluidization is obtained when thevelocity of the fluidizing gas is higher than the transport velocity,and it is characterized in that the pressure gradient along thedirection of transport is a monotonic function of the quantity ofinjected solid, for equal flow rate and density of the fluidizing gas.Contrary to the present invention, in the fluidized-bed technology ofthe known state of the art, the fluidizing-gas velocity is maintainedwell below the transport velocity, in order to avoid phenomena of solidsentrainment and particle carryover. The terms transport velocity andfast fluidization state are well known in the art; for a definitionthereof, see, for example, “D. Geldart, Gas Fluidization Technology,page 155 et seqq., J. Wiley & Sons Ltd., 1986”.

[0034] In the second polymerization zone, where the polymer flows in adensified form under the action of gravity, high values of density ofthe solid are reached (density of the solid=kg of polymer per m³ ofreactor occupied by polymer), which approach the bulk density of thepolymer; a positive gain in pressure can thus be obtained along thedirection of flow, so that it becomes possible to reintroduce thepolymer into the first reaction zone without the help of specialmechanical means. In this way, a “loop” circulation is set up, which isdefined by the balance of pressures between the two polymerization zonesand by the head loss introduced into the system.

[0035] The invention is described with reference to the attachedfigures, which are given for illustrative purposes without limiting theinvention, in which:

[0036]FIG. 1 is a diagrammatic representation of the process accordingto the invention,

[0037]FIG. 2 is a diagrammatic representation of a first embodiment ofthe process according to the invention, and

[0038]FIG. 3 is a diagrammatic representation of a second embodiment ofthe process according to the invention.

[0039] Referring to FIG.1, the growing polymer flows through the firstpolymerization zone 1 under fast fluidization conditions along thedirection of the arrow 14; in the second polymerization zone 2, thegrowing polymer flows in a densified form under the action of gravityalong the direction of the arrow 14′. The two polymerization zones 1 and2 are appropriately interconnected by the sections 3 and 5. The materialbalance is maintained by feeding in monomers and catalysts anddischarging polymer.

[0040] Generally, the condition of fast fluidization in the firstpolymerization zone 1 is established by feeding a gas mixture comprisingone or more α-olefins CH₂=CHR (line 10) to said zone 1; preferably, thefeeding of the gas mixture is effected below the point of reintroductionof the polymer into said first zone 1 by the use, where appropriate, ofgas distributor means, such as, for example, a distributor grid.

[0041] The velocity of the transport gas into the first polymerizationzone is higher than the transport velocity under the operatingconditions and is preferably between 2 and 15 m/s, more preferablybetween 3 and 8 m/s.

[0042] The control of the polymer circulating between the twopolymerization zones can be effected by metering the amount of polymerleaving the second polymerization zone 2, using means suitable forcontrolling the flow of solids, such as, for example, mechanical valves(slide valve, V-ball valve, etc.) or non-mechanical valves (L valve, Jvalve, reverse seal, etc.).

[0043] Generally, the polymer and the gaseous mixture leaving the firstpolymerization zone 1 are conveyed to a solid/gas separation zone 4. Thesolid/gas separation can be effected by using conventional separationmeans such as, for example, a separator of the inertial type orpreferably centrifugal type, or a combination of the two. Thecentrifugal separator (cyclone) can be of the axial, spiral, helical ortangential type.

[0044] From the separation zone 4, the polymer enters the secondpolymerization zone 2. The gaseous mixture leaving the separation zone 4is compressed, cooled and transferred, if appropriate with addition ofmake-up monomers and/or molecular weight regulators, to the firstpolymerization zone 1. This transfer can be effected by means of arecycle line 6 for the gaseous mixture, equipped with means for thecompression 7 and cooling 8 and means for feeding in the monomers andthe molecular weight regulator 13.

[0045] A part of the gaseous mixture leaving the separation zone 4 canbe transferred, after having been compressed, to the connection zone 5via the line 9, in order to facilitate the transfer of polymer from thesecond to the first polymerization zone.

[0046] Preferably, the various catalyst components are fed to the firstpolymerization zone 1, at any point of said first polymerization zone 1.However, they can also be fed at any point of said second polymerizationzone 2. Any type of catalyst used in the polymerization of olefins canbe used in the process of the present invention, since it is notimportant for it to be in any particular physical state, and catalystsin either solid or liquid form can be used, because, in contrast to thegas-phase processes of the known state of the art, the process of thepresent invention does not necessarily require the use of catalysts inwhich at least one component is in a granular form, but can be carriedout with catalysts in which the various components are in solution. Forexample, catalysts based on titanium, chromium, vanadium or zirconiumcan be used either in supported or unsupported form. Examples ofcatalysts which can be used are described in the patents U.S. Pat. No.4,748,272, U.S. Pat. No. 4,302,566, U.S. Pat. No. 4,472,520 and U.S.Pat. No. 4,218,339.

[0047] Particularly suitable are the catalysts of controlled morphology,which are described in the patents U.S. Pat. No. 4,399,054, U.S. Pat.No. 5,139,985, EP-395,083, EP-553,805, EP-553,806 and EP-601,525, and ingeneral catalysts capable of giving polymers in the form of spheroidalparticles having a mean dimension between 0.2 and 5 mm, preferablybetween 0.5 and 3 mm. The process of the present invention is moreoverparticularly suitable for the use of metallocene catalysts, either insolution or supported. The various catalyst components can be introducedat the same point or at different points of the first polymerizationzone.

[0048] The catalyst can be fed in without prior treatment or in aprepolymerized form. Where other polymerization stages are situatedupstream, it is also possible to feed the polymerization zones with acatalyst dispersed in a polymer suspension coming from an upstream bulkreactor, or a catalyst dispersed in a dry polymer coming from anupstream gas-phase reactor.

[0049] The polymer concentration in the reactive zones can be monitoredby the usual methods known in the state of the art, for example bymeasuring the differential pressure between two suitable points alongthe axis of the polymerization zones or measuring the density by nucleardetectors (for example γ-rays).

[0050] The operating parameters such as, for example, the temperatureare those that are usual in gas-phase olefin polymerization processes,for example between 50° C. and 120° C.

[0051] The process according to the present invention has manyadvantages. The loop configuration allows the adoption of relativelysimple reactor geometries. In practice, each reaction zone can bedesigned as a cylindrical reactor of high aspect ratio (height/diameterratio). From the point of view of construction, this particular reactorgeometry allows the adoption of high operating pressures, which are noteconomical in conventional fluidized-bed reactors. The process accordingto the present invention can thus be carried out under operatingpressures of between 0.5 and 10 MPa, preferably between 1.5 and 6 MPa.The consequent high gas density favours both the heat exchange on asingle particle and the overall removal of the heat of reaction. It istherefore possible to choose operating conditions which enhance thereaction kinetics. Moreover, the reactor through which the polymer flowsunder fast fluidization conditions (first polymerization zone) can runcompletely full at polymer concentrations which can reach or exceed 200kg/m³. With the contribution of the second polymerization zone andtaking account of the more favourable kinetic conditions which can beestablished, the process of the present invention makes it possible toobtain specific productivities (hourly output per unit volume of thereactor) which are much higher than the levels obtainable withconventional fluidized-bed technology. It is thus possible to equal oreven to exceed the catalytic yields of conventional gas-phase processes,using polymerization equipment of much more limited dimensions, with asignificant saving in the construction cost of the plant.

[0052] In the process according to the present invention, theentrainment of solids in the gas recycle line at the exit from thesolid/gas separation zone and the possible presence of liquids exitingthe cooler on the same line do not limit the efficiency of the firstpolymerization zone. Even when using gas distributor means such as, forexample, a grid, the transport gas velocities in the plenum below thegrid are still high and such as to ensure the entrainment of droplets ofeven considerable dimensions and of wetted polymer, without stagnantpoints. Given that the transport gas comes into contact with the streamof hot polymer arriving from the second polymerization zone, thevaporization of any liquid is virtually instantaneous. It is thereforepossible to cool the gaseous mixture leaving the solid/gas separationzone to temperatures below the dew point in order to condense part ofthe gases. The gas/liquid mixture which forms is then fed to the firstpolymerization zone where it contributes to heat removal withoutencountering the problems and limits of the known state of the art andwithout requiring the use of the complicated devices proposed to avoidthem. In addition to and/or in replacement of the partial condensationof the recirculating gases, the process of the invention opens a new wayto the removal of the heat of reaction. The characteristic geometry(high surface/volume ratio) of the polymerization zone with fastfluidization makes a significant external surface area available fordirect heat exchange on this zone (and hence with maximum heat transferbetween cooling liquid and reaction system). Where convenient,additional or alternative heat exchange surfaces can be present in theinterior of the reactor. The first polymerization zone can thus beadvantageously cooled with external cooling means. The high turbulenceconnected with the fast fluidization conditions and a high gas densityassure in every case a very high heat transfer coefficient. Anycondensation on the internal walls is continuously removed by the strongradial and axial mixing of the polymer due to the fast fluidizationconditions. Moreover, this characteristic makes the proposed technologysuitable for operation as a second stage fed directly from an upstreambulk reactor. It is also possible to feed part of the make-up monomersin a condensed form without any difficulty. As far as the removal of theheat of reaction is concerned, the capacities offered by the process ofthe invention are superior to those of the known state of the art andthe difficulties inherent in the prior technologies are overcome.Moreover, the volumetric rates of the circulating gas are notnecessarily dependent upon the requirements of heat exchange.

[0053] Advantageously, one or more inert gases are maintained in thepolymerization zones, in such quantities that the sum of the partialpressures of the inert gases is preferably between 5 and 80% of thetotal pressure of the gases. The inert gas can be nitrogen or analiphatic hydrocarbon having 2-6 carbon atoms, preferably propane. Thepresence of the inert gas has numerous advantages, for it makes itpossible to moderate the reaction kinetics while at the same timemaintaining total reaction pressures which are sufficient to keep lowthe head of the circulation compressor and to assure an adequate massflow rate for the heat exchange on the particle in the bed and, throughthe cooler on the circulating gaseous mixture, for the removal of theheat of reaction which has not been removed by the surfaces.

[0054] In the process of the present invention, the presence of theinert gas has further advantages, inasmuch as it makes it possible tolimit the temperature increase in the second polymerization zone, whichruns in an essentially adiabatic mode, and also makes it possible tocontrol the breadth of the molecular weight distribution of the polymer,particularly in the polymerization of ethylene. This is because, asalready stated, the polymer flows vertically down through the secondpolymerization zone in plug flow (packed flow mode), surrounded bylimited quantities of entrained gas. As is known, the molecular weightof the polymer in ethylene polymerization is controlled by thehydrogen/ethylene ratio in the gas phase and, to a lesser extent, by thetemperature. In the presence of inerts, given that the reaction consumesethylene but hydrogen only to a marginal extent, the ethylene/hydrogenratio decreases along the axis of the polymer flow in the direction ofmovement, causing growth of polymeric fractions on the same particlewith decreasing molecular weights. The temperature rise due to thereaction adds to this effect. It is therefore possible, by means of anappropriate balancing of the gas composition and the residence times inthe two polymerization zones, to control in an effective manner thebroadening of the molecular weight distribution of the polymers while atthe same time maintaining maximum homogeneity of the product.

[0055] Conversely, if it is desired to produce polymers with a narrowmolecular weight distribution, the mechanism described above can berestricted or avoided by proper selection of the reaction conditions,for example by limiting the amount of inert gas or feeding anappropriate quantity of reaction gas and/or make-up monomer(s) atsuitable positions in the second polymerization zone. Advantageously,the gas to be fed to the second polymerization zone can be taken fromthe gaseous mixture leaving the solid/gas separation zone, after thishas been compressed. The quantity of gas introduced is preferably fixedwithin values such that the relative velocity of the injected gas withrespect to the flowing solid velocity, is kept below the minimumfluidization velocity characteristic of the solid/gas system present insaid second polymerization zone. Under these conditions, the downwardflow of polymer is substantially not disturbed. The operationalflexibility of the process of the invention is therefore total, theproduction of polymers of different molecular weight distribution beingcontrollable by the gas composition and, if needed, by simple closing oropening of a valve on a gas line.

[0056] Advantageously, the polymer can be discharged from zones wherethe solids density is higher, for example from suitable points in thesecond polymerization zone where large amounts of densified flowingpolymer are available, in order to minimise the quantity of entrainedgas. By inserting a controlled valve at a suitable point upstream of theexit region of the polymer from the second polymerization zone, itbecomes possible continuously to control the withdrawal of the polymerproduced. The amount of gas accompanying the polymer is extremely smalland only slightly greater than can be achieved by the device ofinterposing a series of hoppers in alternating intermittent operation.In this way, all the limitations of the discharge systems of the knownstate of the art are overcome, with respect to both the quantity ofentrained gas and the nature of the discharged products.

[0057] As already stated, the process of the present invention can becombined with the conventional technologies in a sequential multi-stageprocess in which, upstream or downstream of a polymerization sectionoperated according to the present invention, there are one or morepolymerization stages using conventional technologies (in bulk or in thegas phase, either in a fluidized bed or a stirred bed). Multi-stageprocesses, wherein two or more stages are carried out with the procedureof the present invention, are also possible.

[0058] It is moreover possible to combine the process according to thepresent invention with the conventional fluidized-bed gas-phasetechnologies by interposing, between the two polymerization zones asdefined in the present invention, a polymerization zone using a fluidbubble bed, i.e. with fluidization gas velocities higher than theminimum fluidization velocity and lower than the transport velocity,while always maintaining the loop circulation characteristic of theprocess of the present invention. For example, one possible embodimentprovides that the second polymerization zone consists of a first and asecond section. In the first (with respect to the downward flow of thepolymer) of said sections a fluidized bed is maintained by appropriatelyfeeding and distributing gases; in the second section, appropriatelyconnected to the first one, the polymer flows in densified form bygravity. From the second section the polymer is reintroduced into thefirst polymerization zone, maintaining the loop circulation. With anappropriate dimensioning of the various zones, it becomes possible toachieve a braodening of the molecular weight distribution of thepolymer, while retaining all the advantages described above. The aboveexample is only one of the possible embodiments of the process of theinvention, which, in its general definition, comprises at least a fastfluidization zone interconnected with a zone where the polymer flows indensified form by gravity.

[0059] The process of the present invention is applicable to thepreparation of a large number of olefin polymers without thedisadvantages described above. Examples of polymers which can beobtained are:

[0060] high-density polyethylenes (HDPEs having relative densitieshigher than 0.940) including ethylene homopolymers and ethylenecopolymers with α-olefins having 3 to 12 carbon atoms;

[0061] linear polyethylenes of low density (LLDPEs having relativedensities lower than 0.940) and of very low density and ultra lowdensity (VLDPEs and ULDPEs having relative densities lower than 0.920down to 0.880) consisting of ethylene copolymers with one or moreα-olefins having 3 to 12 carbon atoms;

[0062] elastomeric terpolymers of ethylene and propylene with minorproportions of diene or elastomeric copolymers of ethylene and propylenewith a content of units derived from ethylene of between about 30 and70% by weight;

[0063] isotactic polypropylene and crystalline copolymers of propyleneand ethylene and/or other α-olefins having a content of units derivedfrom propylene of more than 85% by weight;

[0064] heterophasic propylene polymers obtained by sequentialpolymerization of propylene and mixtures of propylene with ethyleneand/or other α-olefins;

[0065] atactic polypropylene and amorphous copolymers of propylene andethylene and/or other α-olefins containing more than 70% by weight ofunits derived from propylene;

[0066] poly-α-olefins, such as, e.g., poly-1-butene,poly-4-methyl-1-pentene;

[0067] polybutadiene and other polydiene rubbers.

[0068] A further aspect of the present invention relates to an apparatusfor the gas-phase polymerization of α-olefins. The apparatus of theinvention comprises a first vertical cylindrical reactor 20 equippedwith a catalyst feedline 34, and a second vertical cylindrical reactor30 equipped with a polymer discharge system 23, and is characterized inthat: the upper region of the first reactor 20 is connected by a firstline 21 to a solid/gas separator 22 which in turn is connected to theupper region of the second reactor 30; the lower region of the secondreactor 30 is connected by a, second line 31 to the lower region of thefirst reactor 20; and the solid/gas separator 22 is connected by meansof a recycle line for the gaseous mixture 36 to the first reactor 20 ina region 37 at the bottom of said first reactor 20 below the point ofentry of the second line 31.

[0069] Preferably, the first reactor 20 is equipped with gas distributormeans 33, for example a grid, located between the point of entry of thesecond line 31 and the region 37 at the bottom of this reactor. As analternative, with reference to Fig. 3, the gas distributor means in thefirst reactor 60 can be replaced by a cylindrical line 65, through whichthe gas flows at high velocity and which is connected to the reactor 60by a frustoconical section 62 whose angle of inclination to the verticalis preferably smaller than 45° and more preferably between 30 and 10°.Advantageously both the catalyst (thorough line 66) and the polymercoming from the second reactor 70 through the line 77 can be conveyedthrough this frustoconical connection.

[0070] A first valve 24 for controlling the polymer flow rate isgenerally inserted between the second reactor 30 and the second line 31.This valve 24 can be either of the mechanical or of the non-mechanicaltype.

[0071] In the case where gas distributor means 33 are present, some orall the components of the catalyst can advantageously be injected via athird line 32 into said first reactor 20 at a point above the gasdistributor means.

[0072] Advantageously, the recycle line for the gaseous mixture 36 isequipped with a compressor 26, a cooling system 27 and systems forintroducing, together or separated, monomers 28 and molecular weightregulator 29. Two cooling systems, one upstream and one downstream thecompressor, can be present.

[0073] Preferably, the first line 21 leaves the upper region of thefirst reactor laterally, it having been observed that a lateral exit ofthe solid/gas mixture from the first reactor 20 contributes in asubstantial way to the dynamic stability of the entire reaction system.

[0074] The upper region of the first reactor 20 can have a cylindricalshape with a diameter equal to that of the reactor or preferably can beof frustoconical geometry with the broad end uppermost.

[0075] The first line 21 can be horizontal or have a slope in thedirection of gravity in order to facilitate discharge of polymer (seethe configuration of the line 71 in FIG. 3). The second line 31 canappropriately be inclined downwards and can be connected (at a pointimmediately downstream of the first valve 24) via a line 25 to the gasrecirculation line 36 at a point downstream of the compressor 26. Inthis way, the flow of polymer is assisted by the stream of gas underpressure coming from the recycle line, avoiding stagnant zones ofpolymer in the line itself and at the point of introduction into thereactor 20.

[0076] The system of connection between the lower regions of thereactors can also be of the type described in FIG. 3, in which thecirculation of the polymer is obtained by a pneumatic L valve 74operated by the gas taken from the recycle line through the line 75. TheL valve is connected to a line 77 which leads into the first reactor 60,said line 77 being connected via the line 76 to the recycle line 81.Through this line, the polymer is carried back to the interior of thereactor 60 by an appropriate stream of gas coming from the line 76.

[0077] The first reactor 20 can advantageously be equipped with externalcooling means 35, such as wall heat exchangers.

[0078] Two possible embodiments of the invention are illustrated in FIG.2 and in FIG. 3. These embodiments have a purely illustrative purposeand do not limit the invention.

[0079] With reference to FIG. 2, 20 represents the first reactoroperating under fast fluidization conditions and 30 represents thesecond reactor through which the polymer flows in a densified form underthe action of gravity; 21 and 31 are lines connecting the upper andlower regions of the two reactors; 34 is the catalyst feedline; 22 is asolid/gas separator; 23 is a polymer discharge system; 36 is the recycleline for the gaseous mixture which connects said separator to a region37 at the bottom of the first reactor; 24 is a control valve forcontrolling the polymer flow rate; 33 is a gas distributor device; 32 isa line for feeding the catalyst; 26 is a compressor and 27 is a coolingsystem for the recycling gas mixture; 28 and 29 are systems for feedingmonomers and molecular weight regulator; 25 is a line which connects therecycle line 36 to the line 31; 35 is the external cooling system of thefirst reactor 20.

[0080] With reference to FIG. 3, 60 represents the first reactoroperating under fast fluidization conditions and 70 represents thesecond reactor through which the polymer flows in a densified form underthe action of gravity; 71 and 77 are lines connecting the upper andlower regions of the two reactors; 66 is the catalyst feedline; 72 is asolid/gas separator; 73 is the polymer discharge system; 81 is therecycle line for the gaseous mixture, which connects said separator 72to a line 65 connected to the base of the first reactor 60 by afrustoconical section 62; 74 is an L valve for controlling the polymerflow rate; 79 is a compressor and 80 is a cooling system for the gaseousrecycle mixture; 63 and 64 are feed systems for monomers and molecularweight regulator; 75 is a line which connects the recycle line 81 to theL valve 74; 76 is a line which connects the recycle line 81 to the line77; 78 is a line which connects the recycle line 81 to a region at thebottom of the second reactor 70; 61 is the external cooling system forthe first reactor 20.

[0081] The following examples will further illustrate the presentinvention without limiting its scope.

EXAMPLES General Polymerization Conditions.

[0082] Polymerizations were carried out in continuous in a plant whichcomprised a precontacting section, where the various catalyst componentwere premixed, a prepolymerization section, and a gas-phasepolymerization section carried out in a reactor of the type described inFIG. 2.

[0083] A solid catalyst component prepared according to the proceduredescribed in Example 3 of EP-A-395083, triethylaluminum (TEAL) and asilane compound were precontacted in hexane at 10° C. for 10 minutes inthe precontacting vessel. The activated catalyst was fed to theprepolymerization section were propylene was polymerized in slurry usingpropane as dispersing medium. Monomer feed and residence time wereadjusted so as to obtain the desired prepolymerization yields, in termsof g of polymer per g of solid catalyst component.

[0084] The prepolymer was continuously fed to the gas phasepolymerization apparatus. The apparatus, which is described withreference to FIG. 2, consisted of two cylindrical reactors 20 and 30,connected by pipes 21 and 31. Reactor 20 was equipped with a heatexchanger 35. Fast fluidization in the reactor 20 was achieved byrecycling gas from the gas/solid separator 22 to the bottom of thereactor 20, via the gas-recycle line 36. No gas-distribution means wereused, the recycle gases being directly fed to a region 37 at the bottomof the reactor 20, below the point of entry of pipe 31. The gas-recycleline was equipped with a compressor 26 and a heat exchanger 27. Theprepolymer slurry reactor was fed to the reactor 20 at a pointimmediately above the point of entry of pipe 31. Circulation of polymerwas controlled via a L valve 24 operated by a stream of gas 25 takenfrom the recycle line 36. Make-up monomers were fed to the recycle line36. The polymer produced was continuously discharged from the reactor30, via pipe 23. Total volume of the apparatus (i.e. reactors 20 and 30plus connection zones 21 and 31) was 250 1.

EXAMPLE 1

[0085] Polypropylene was prepared using a catalyst comprisingdicyclopentyl-dimethoxy-silane (DCPMS) as silane compound. In thegas-phase polymerization step, propane was used as inert gas. Mainoperating condition. Precontacting step. TEAL/solid component (wt.) 8TEAL/DCPMS (wt.) 3 Prepolymerization step. Yield (g/g) 100 Gas-PhasePolymerization. Temperature (° C.) 85 Pressure (barg) 25 Propylene (%mol) 91 Propane (% mol) 8 Hydrogen (% mol) 1 Specific productivity (Kg/h· m³) 140 Product Characteristics. Bulk Density (kg/l) 0.45

EXAMPLE 2

[0086] Hexene-modified LLDPE was prepared using a catalyst comprisingcyclohexyl-methyl-dimethoxy-silane (CMMS) as silane compound. In thegas-phase polymerization step, propane was used as inert gas. Mainoperating condition. Precontacting step. TEAL/Ti (wt.) 120 TEAL/CMMS(wt.) 20 Prepolymerization step. Yield (g/g) 400 Gas-PhasePolymerization. Temperature (° C.) 75 Pressure (barg) 24 Ethylene (%mol) 15 1-Hexene (% mol) 1.5 Hydrogen (% mol) 3 Propane (% mol) 80.5Specific productivity (Kg/h · m³) 80 Product Characteristics. Melt IndexE (g/10 min) 1.4 Density (g/cm³) 0.908

[0087] The above reported temperature was measured at top of the reactor30. The dew point of the gaseous mixture at the operating pressure is66° C. Cooling fluid was circulated in the heat exchanger 35 in such away to obtain a temperature of 63° C. on the surface of reactor 20.Under these conditions, the gaseous mixture partially condensed on thewall of the reactor, thus contributing to remove the heat of reaction.No problems of fouling occurred during operation.

Claims
 1. Process for the gas-phase polymerization of α-olefins CH₂=CHR,where R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms,carried out in a first and in a second interconnected polymerizationzones, to which one or more of said α-olefins are fed in the presence ofa catalyst under reaction conditions and from which the polymer productis discharged, wherein the growing polymer particles flow through thefirst of said polymerization zones under fast fluidization conditions,leave said first polymerization zone and enter the second of saidpolymerization zones through which they flow in a densified form underthe action of gravity, leave said second polymerization zone and arereintroduced into said first polymerization zone, thus establishing acirculation of polymer between the two polymerization zones.
 2. Processaccording to claim 1, wherein said fast fluidization conditions areestablished by feeding a gaseous mixture comprising one or more of saidα-olefins CH₂=CHR to said first polymerization zone.
 3. Processaccording to claim 2, wherein said gaseous mixture is fed to said firstpolymerization zone in a region below the point of reintroduction of thepolymer into said first polymerization zone.
 4. Process according toclaim 3, wherein the feeding of said gaseous mixture is carried out bymeans of gas distributor means.
 5. Process according to claim 2, whereinthe polymer and the gaseous mixture leaving said first polymerizationzone are conveyed to a solid/gas separation zone and the polymer leavingsaid solid/gas separation zone enters said second polymerization zone.6. Process according to claim 1, wherein the control of the polymercirculating between the said two polymerization zones is effected bymetering the quantity of polymer leaving said second polymerizationzone.
 7. Process according to claim 1, wherein the polymer produced iswithdrawn continuously from said second polymerization zone.
 8. Processaccording to claim 1, wherein the catalyst components are fed to saidfirst polymerization zone.
 9. Process according to claim 1, wherein anyof the reaction zones is fed with a catalyst in a prepolymerized form.10. Process according to claim 1, wherein any of the reaction zones isfed with a catalyst dispersed in a polymer slurry.
 11. Process accordingto claim 1, wherein any of the reaction zones is fed with a catalystdispersed in a dry polymer.
 12. Process according to claim 5, whereinthe gaseous mixture leaving said solid/gas separation is compressed,cooled and transferred, if appropriate with addition of make-upmonomers, to said first polymerization zone.
 13. Process according toclaim 5, wherein part of the gaseous mixture leaving the solid/gasseparation zone is used for transferring the polymer from said secondzone to said first polymerization zone.
 14. Process according to claim5, wherein part of the gaseous mixture leaving said solid/gas separationzone is compressed and transferred to said second polymerization zone inthe vicinity of the region where the polymer leaves said second zone.15. Process according to claim 12, wherein the gaseous mixture leavingsaid solid/gas separation zone is cooled to temperatures below the dewpoint.
 16. Process according to claim 1, wherein said firstpolymerization zone is cooled by external cooling means.
 17. Processaccording to claim 1, wherein the make-up monomer or monomers are fed inan at least partially condensed form to said first polymerization zone.18. Process according to claim 2, wherein the velocity of the fluidizinggas into said first polymerization zone is between 2 and 15 M/s,preferably between 3 and 8 m/s.
 19. Process according to claim 1,wherein the polymer is in the form of spheroidal particles having meandimensions of between 0.2 and 5 mm, preferably between 0.5 and 3 mm. 20.Process according to claim 1, wherein the working pressure is between0.5 and 10 MPa, preferably between 1.5 and 6 MPa.
 21. Process accordingto claim 1, wherein one or more inert gases are present in saidpolymerization zones at partial pressures of between 5 and 80% of thetotal pressure of the gases.
 22. Process according to claim 21, whereinthe inert gas is nitrogen or an aliphatic hydrocarbon having 2-6 carbonatoms, preferably propane.
 23. Process according to claim 1, wherein anintermediate polymerization zone, operating with a fluid bed, isinterposed between said first and said second polymerization zones. 24.Apparatus for the gas-phase polymerization of α-olefins, comprising: afirst vertical cylindrical reactor (20) equipped with a catalystfeedline (34); and a second vertical cylindrical reactor (30) equippedwith a polymer discharge system (23); the upper region of said firstreactor (20) being connected by a first line (21) to a solid/gasseparator (22) which is in turn connected to the upper region of saidsecond reactor (30); the lower region of said second reactor (30) beingconnected by a second line (31) to the lower region of said firstreactor (20); and said solid/gas separator (22) being connected by meansof a recirculation line for the gaseous mixture (36) to said firstreactor (20) in a region (37) at the bottom of said first reactor (20)below the point of entry of said second line (31).
 25. Apparatusaccording to claim 24, wherein said first reactor (20) is equipped withgas distributor means (33) located between the point of entry of saidsecond line (31) and said region (37) at the bottom of said firstreactor (20).
 26. Apparatus according to claim 24, wherein a firstcontrol valve (24) for controlling the polymer flow rate is interposedbetween said second reactor (30) and said second line (31). 27.Apparatus according to claim 26, wherein said first valve (24) is amechanical valve.
 28. Apparatus according to claim 26, wherein saidfirst valve (24) is a non-mechanical valve.
 29. Apparatus according toclaim 25, wherein said catalyst feedline (34) is connected via a thirdline (32) to said first reactor (20) at a point above said gasdistributor means (33).
 30. Apparatus according to claim 24, whereinsaid recirculation line for the gaseous mixture (36) is equipped with acompressor (26), a cooling system (27) and systems for introducingmonomers (28) and molecular weight regulator (29).
 31. Apparatusaccording to claim 24, wherein said first line (21) leaves the upperregion of said first reactor (20) laterally.
 32. Apparatus according toclaim 24, wherein the upper region of said first reactor is offrustoconical geometry with the broad end uppermost.
 33. Apparatusaccording to claim 30, wherein said recirculation line for the gaseousmixture (36) is connected, at a point downstream of said compressor(26), via a line (25) to said second line (31).
 34. Apparatus accordingto claim 24, wherein said first reactor (20) is equipped with externalcooling means (35).